What mechanisms drive neural induction and neural determination in urodeles?

ABSTRACT In our laboratory we use Urodeles (Pleurodeles waltl) and Anurans (Xenopus laevis) to perform comparative studies on neural determination. Urodeles are a good embryological system to study early events in ontogenesis since they present several advantages: slow development, external localization of chordamesoderm at the beginning of gastrulation, large size of cells, diploid genome, etc. I have focused this overview-report on the main findings on Pleurodeles neurogenesis. The determination of the two neural lineages (neuronal and astroglial) appears during gastrulation as a consequence of (a) permissive event(s) activated through a Ca(++)-dependent transducing pathway. This signaling-pathway involves L-type Ca++ channels. The activation of this Ca++ transduction route is sufficient to activate both neuronal and glial structural specific genes, via direct activation of "immediate early genes". The specification of neuronal functional differentiation depends on additional factors of chordamesoderm origin acting during gastrulation and later on. At the early neurula stage, in the neural plate, 20% of progenitor cells present a neuronal fate, 80% are at least bipotential and generate mixed clones (neurons and astrogliocytes). The issue of the state of "commitment" of the precursor cells (competent ectoderm) and the identification of specifying molecules (from Spemann organizer) are underway in Pleurodeles and Xenopus.